Hitomi Mikami

Abundances of HCN and HNC in Dark Cloud Cores

We have determined the abundances of HCN and HNC toward 19 nearby dark cloud cores by observations of optically thin H13CN (J = 1-0) and HN13C (J = 1-0) lines. The column density of HCN is found to be correlated with that of HNC. The abundance ratio of [HNC]/[HCN] is determined to be 0.54-4.5 in the observed dark cloud cores. These results are consistent with the idea that HCN and HNC are produced mainly by a recombination reaction of HCNH+ with electrons in dark cloud cores. Furthermore, the [HNC]/[HCN] ratio does not show any significant differences between star-forming cores and starless cores. The HCN and HNC abundances are compared with those for the OMC-1 cores previously reported. Although the abundances of HCN in the OMC-1 cores are comparable to those in the dark cloud cores, the abundances of HNC in OMC-1 are 1-2 orders of magnitude less than those in dark cloud cores. It is suggested that HNC is destroyed by neutral-neutral reactions in high kinetic temperature regions.

Molecular cloud cores in the Orion A cloud. I: Nobeyama CS (1-0) survey

A first high-resolution survey of molecular cloud cores in the Orion A giant molecular cloud is reported. We identified 125 molecular cloud cores from an analysis of the spatial and velocity distribution of the CS (1-0) emission. The cores are generally elongated along the filamentary molecular cloud, and the axial ratio is about 0.5. The mass spectrum index of the cores is -1.6 for M≥50 M ○ .. The physical properties of the cores identified in Orion are compared with those of cores in dark clouds reported in the literature. The average radius of the cores in the Orion A cloud, 0.16 pc, is comparable to that of the cores in dark clouds

Detection of SiO emission in the L1157 dark cloud

Intense thermal emission lines of SiO (v = 0, J = 1-0 and 2-1) are detected in a low-mass star-forming region, L1157. The emission is confined to a compact region toward the blue lobe of the CO outflow, where shock heating due to interaction between the outflow and dense gas has been found. In contrast, the emission of SiO is not detected toward the IRAS source and the red lobe. The distribution of SiO is different from that of CS; the intense CS line is observed in the vicinity of the IRAS source. The peak fractional abundance of SiO is estimated to be about 10 -7

The outflow in the L1157 dark cloud - Evidence for shock heating of the interacting gas

In the L1157 dark cloud, a well-collimated bipolar CO outflow associated with a cold IRAS source 20386+6751 have been discovered. The gas kinetic temperature toward the blue lobe of the outflow rises to-30 K from the temperature of the surrounding gas (≤10 K); this high temperature region is very localized with the blue lobe. The HCO + , HCN, and NH3 lines show blueshifted and broad-line profiles toward the blue CO lobe. Furthermore, their distributions are similar to that of the blue lobe

The Ortho-to-Para Ratio of Ammonia in the L1157 Outflow*

We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of the L1157 outflow by observing the six metastable inversion lines from &parl0;J,K&parr0;=&parl0;1,1&parr0; to (6, 6). The highly excited (5, 5) and (6, 6) lines were first detected in the low-mass star-forming regions. The rotational temperature derived from the ratio of four transition lines from (3, 3) to (6, 6) is 130-140 K, suggesting that the blueshifted gas is heated by a factor of approximately 10 as compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in the blueshifted gas is estimated to be 1.3-1.7, which is higher than the statistical equilibrium value. This ratio provides us with evidence that the NH3 molecules have been evaporated from dust grains with the formation temperature between 18 and 25 K. It is most likely that the NH3 molecules on dust grains have been released into the gas phase through the passage of strong shock waves produced by the outflow. Such a scenario is supported by the fact that the ammonia abundance in the blueshifted gas is enhanced by a factor of approximately 5 with respect to the dense quiescent gas.

THE MICROWAVE SPECTRUM OF THE DIHYDRONITROSYL RADICAL, H2NO (2B1)

The microwave rotational spectrum of the H2NO radical in the X 2B1 ground vibronic state was observed by using a source‐modulation spectrometer and a 1 m long free‐space cell. The H2NO radical was generated in the cell by a reaction of hydroxylamine (NH2OH) with the products of 2450 MHz discharge in CF4. The spectral lines of H2NO were observed for the a‐type R‐branch transitions of N=1←0 to N=6←5, most of which showed resolved hyperfine structures due to the hydrogen and nitrogen nuclei. The 19 molecular constants including the hyperfine coupling constants of both the nuclei were precisely determined by the least‐squares fit of 129 observed lines. The observed hyperfine structure shows that the H2NO radical in the ground electronic state is essentially planar with C2v symmetry. The nitrogen hyperfine coupling constants determined indicate that an unpaired electron occupies the pπ orbital perpendicular to the molecular plane, and the electronic wave function in the ground electronic state belongs to 2B1 ...

A Comparison of the Spatial Distribution of H13CO+, CH3OH, and C34S Emission and Its Implication in Heiles Cloud 2

We have mapped the Heiles cloud 2 region in the Taurus molecular cloud complex with H13CO+ (J = 1-0), CH3OH (JK = 20-10 A+), and C34S (J = 2-1) lines. Dense gas traced by the mapped lines with critical densities higher than 104 cm-3 is concentrated in four condensations, that is, the TMC 1 and TMC 1C filaments, L1527, and TMC 1A. We have found that the three emission lines have remarkably different spatial distributions. The H13CO+ emission traces well dense cores harboring protostars, while the CH3OH emission is weak toward the protostars and is rather enhanced toward cores without protostars. We found that there are two starless cores with enhanced CH3OH emission at the northwestern ends of the TMC 1 and TMC 1C filaments, toward which the H13CO+ emission is barely seen. On the basis of the analyses using the large velocity gradient (LVG) model, we show that the CH3OH abundance relative to H13CO+ is enhanced by up to 1 order of magnitude in the cores without protostars. The C34S abundance relative to H13CO+ also shows a similar trend to that of CH3OH. Such an abundance variation between H13CO+ and CH3OH and C34S can be explained in the scheme of time-dependent gas-phase chemical evolution, which predicts that CH3OH and C34S are abundant in the early stages of chemical evolution and become deficient in the later stages. A comparison of the spatial-velocity structures in TMC 1 observed with the three molecular lines suggests that this cloud consists of multiple components with different velocities and different chemical compositions along the line of sight.